PURPOSE: To prevent the generation of weld defects and to produce a welded H- beam having high joint strength and excellent quality by disposing insulating ferroelectric magnetic materials on both sides of a high-frequency current flow path on a web side to accelerate the heating of the part to be welded of a flange.

CONSTITUTION: The insulating ferroelectric magnetic materials 6, 6 are disposed on both the right and left side of the high-frequency current flow path of the web 1. The spread of magnetic lines B of force to the right and left is suppressed by the effect of magnetic focusing so that the magnetic lines of force are changed to a longitudinally long shape and that the magnetic flux of a high density acts on the part 2' to be welded of the flange 2 in a narrow range. The electric current of a high density is, therefore, induced concentrically to the part 2a to be welded and makes an effective contribution to the heating to the flange 2 side. The difference in the heating state between the web 1 and the flange 2 is corrected and the heating on the flange 2 side is selectively accelerated according to the above-mentioned method; therefore, the melting thereof is surely assured.